Geophysical surveys are often used for oil and gas exploration in geophysical formations, which may be located below marine environments. Various types of signal sources and sensors may be used in different types of geophysical surveys. For example, seismic geophysical surveys are based on the use of acoustic waves. In such a survey, a vessel may tow an acoustic source (e.g., an air gun or a marine vibrator) and a plurality of streamers along which a number of sensors (e.g., pressure sensors such as hydrophones, and/or particle velocity sensors such as accelerometers) are located. Acoustic waves generated by the source may then be transmitted into the earth's crust and then reflected back and captured at the sensors. Acoustic waves received during a marine seismic survey may be analyzed to locate hydrocarbon-bearing geological structures, and thus determine where deposits of oil and natural gas may be located.
During the course of surveying, the acoustic sources may develop various types of mechanical problems. For example, sealing washers may degrade with time, leading to air-leakage into the water, which may change the acoustic output from the air gun in an undesirable way. Further, air guns may simply fail during surveying. Generally, the only production tests currently done to check for air gun faults are “bubble tests.” As discussed in more detail below, a bubble test typically includes actuating an air gun and measuring various parameters of the response. Typically, the measuring may be performed via an air gun hydrophone located proximate to the actuated air gun.
The current bubble test methodology is generally time-consuming, typically taking well over an hour, and therefore costly. Accordingly, these bubble tests are usually only done occasionally. Furthermore, bubble tests, as done today, are primarily used to verify air gun volumes (occasionally mistakes can be made when building the array, such that the actual volumes built into the array can be different than those specified), rather than to prevent failures before they occur. According to this disclosure, frequent bubble tests (e.g., every survey line) are possible at minimal cost. This data may allow the performance of the air guns to be monitored, which may prevent air gun failures and thus reduce downtime caused by failing air guns. Some embodiments of this disclosure are extremely efficient and can be carried out on the timescale of one shot record or a few shot records. A “shot record” is defined as the time extending from the actuation of an air gun to the end of recoding of data for that actuation. Typically, a shot record may begin concurrently with the actuation of an air gun, or it may in some embodiments begin slightly before the firing of the air gun—for example, about 50 milliseconds before the firing of the air gun. The shot record typically ends when sufficient time has elapsed such that the desired data has been captured.
The current practice in a bubble test is to fire one air gun per shot record (typically several seconds long) to ensure that interaction between the air gun signals does not occur. A typical length for a shot record in some embodiments is about 10 seconds. There are two main factors that determine this length. Firstly, it is typically desirable for it to be greater than the time it takes for the acoustic energy to travel from the source, through the earth to the deepest point of interest in the earth, and then back to the receiver array. Secondly, for sources such as the air gun, the shot record may advantageously be greater than the time it takes to re-charge the air in the sources. Since the shot records are typically of a similar length during a bubble test, the full bubble test for the array (or for two arrays in “flip-flop” shooting) can thus last several hours, during which normal seismic operations cannot take place. The main purpose of the bubble test is to determine that the actual air gun volumes are indeed the same as their nominal volumes. However, because the bubble test takes so long, it is typically carried out very rarely—for example, before the start of a survey and perhaps a few times during the survey.